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Raft analysis : Solution method

Raft & piled-raft analysis is an iterative process linking the linear static analysis solver for the structure analysis with the PDisp analysis solver for the soil settlement analysis. The process iterates until the difference of the nodal displacements and soil settlements for all the interaction nodes are smaller than the specified tolerance.

The first stage in a raft analysis is a linear static analysis. The soil is represented by support springs of the interaction nodes. For raft nodes, only vertical interaction is considered and for pile nodes, the interaction in all three directions will be considered.

After linear static analysis, the support spring forces are known, so the pressures on soil can be calculated using the nodal interaction areas. Then, a PDisp soil settlement analysis is carried out to obtain soil settlements. From these settlements and the spring forces, new support stiffness can be calculated for the interaction nodes. For pile interaction nodes, the reaction forces from soil will be calculated based on the differences of pile displacements and soil settlements using the data defined in pile-soil interaction table and pile-soil interaction coefficient (PSIC) curves. This iteration continues until the raft and pile displacements and soil settlements are compatible, i.e. differences of soil settlements and raft displacements are smaller than the predefined tolerance.

If maximum pressure of soil has been specified for the raft interaction nodes and the maximum pressure has been reached, the check of the compatibility of raft displacements and soil settlements will be ignored, i.e. the raft displacements and the soil settlements are allowed to be different.

By default, the temporary support springs used during the analysis will be deleted after the analysis is converged or cancelled. However, if the Save soil stiffness box is checked in the analysis wizard, the support spring stiffness at the last iteration will be kept, therefore the Raft model will be modified.